This invention relates to electronic displays, and to point to line, line to area or point to area converters for illuminating displays.
A variety of devices have been proposed for illuminating electronic displays. These devices include backlighting panels, front lighting panels, concentrators, reflectors, structured-surface films and other optical devices for redirecting, collimating, distributing or otherwise manipulating light. Efficient use of the light is particularly important in battery powered electronic displays such as those used in cell phones, personal digital assistants and laptop computers.
U.S. Pat. No. 4,751,615 (Abrams) shows a tapering wedge transparent page lighting device. U.S. Pat. No. 4,811,507 (Blanchet) shows a front light illuminating panel having light-emitting striations whose depth increases with distance from the light source. U.S. Pat. Nos. 5,005,108 (Pristash et al.); 5,050,946 (Hathaway et al.) and 5,594,830 (Winston et al.) show structured-surface panel illuminators whose panel depth varies along the length of the panel. U.S. Pat. Nos. 4,799,137 (Aho); 4,874,228 (Aho et al.); 5,054,885 (Melby); and 5,190,370 (Miller et al.) show various light fixtures having inclined or curved prismatic structured surface films.
U.S. Pat. Nos. 5,359,691 and 5,608,837 (both to Tai et al.); U.S. Pat. No. 5,485,354 (Ciupke et al.); and U.S. Pat. Nos. 5,608,550 and 5,894,539 (both to Epstein et al.) show structured-surface backlighting or front lighting devices having triangular microprisms or microgrooves separated by flat land areas or lands. European Patent Application EP 0 802 446 Al (Seiko Epson) shows a structured-surface backlighting device having rectangular projections separated by lands. U.S. Pat. No. 5,671,994 (Tai et al) shows a structured-surface front lighting device having trapezoidal projections separated by lands.
U.S. Pat. No. 5,396,350 (Beeson et al.); U.S. Pat. Nos. 5,428,468 and 5,555,109 (both to Zimmerman et al.) and U.S. Pat. No. 5,555,329 (Kuper et al.) show various structured-surface backlighting devices having an array of microprisms with tilted sidewalls.
U.S. Pat. Nos. 5,506,929, 5,668,913 and 5,835,661 (all to Tai et al.) and U.S. Pat. No. 5,613,751 (Parker et al.) show light-expanding systems for converting a point light source into a linear or planar light beam.
Although a number of backlight and front light guide devices have been proposed, there is an ongoing need for more efficient designs and for reduced power consumption.
Many current designs do not use all of the light supplied by the light source. If such unused light could be channeled to the display, then power consumption could be further reduced and display brightness could be increased.
Several of the above-mentioned designs have a more or less constant thickness and a generally flat structured surface. These designs may fail to extract light that travels to the end of the device without striking (and thereby being reflected from or extracted by) the structured surface.
A wedge-shaped design can reduce the amount of (and thus the loss of) light that travels the length of the device without being reflected or extracted. However, some current wedge designs are difficult to manufacture. For example if the device tapers to a thin edge at its distal end, the edge can be difficult to mold and may fracture or craze in use. If instead a mirrored surface is employed at the distal end, some light will be lost due to inefficient reflection at the mirror, and other light will be lost due to reduced efficiency of extraction for reflected light returning towards the input end of the device. Addition of a mirror can also increase manufacturing costs or lower manufacturing yields.
The efficiency and evenness of light extraction by wedge-shaped backlight or front light guide devices generally is governed in part by the gross overall dimensions and shape of the wedge, and by the fine-featured (usually microscopic) dimensions and shapes of any structured-surface features on the device. For example, the design shown in the above-mentioned Hathaway et al. patent has an inclined structured-surface face in the form of a series of descending steps. Each step has a flat land area and an inclined facet. Light typically is extracted from such a device at the facets. Light typically is redistributed within such a device by total internal reflection or xe2x80x9cTIRxe2x80x9d of light striking the lands or by TIR of light striking an opposing surface of the device. The reflected light travels toward the narrow or distal end of the device and may eventually emerge from the device by extraction at a facet. However, some of the reflected light will be lost at the distal end of the device and thus is not available for display illumination.
Several of the other constant thickness or wedge-shaped backlight or light guide devices mentioned above can also be regarded as having facets and lands, although other terms may have been used to describe the parts of these devices. In general, the overall shape of the backlight or light guide and the geometry of the structured surface will affect the ratio of extracted light to reflected light for these devices as well.
We have found that by using a wedge design having a structured surface that includes risers which serve to increase the portion of the facet area available for light extraction, we can obtain improvements in uniformity of illumination, control over light extraction, efficiency, or in more than one or even all of these factors. The present invention provides, in one aspect, a light extraction device having:
a) a light input end;
b) a generally planar light output surface that can deliver light to a display; and
c) a structured light-redirecting surface located generally opposite the light output surface;
wherein the light output surface and light-redirecting surface define a generally wedge-shaped profile that decreases in thickness from the light input end towards the center of the device; the light-redirecting surface reflects light towards the light output surface and thence to the display; and the light-redirecting surface comprises a plurality of projections comprising a riser segment, plateau segment, and facet segment, with land segments flanking the projections.
Without risers, the amount of extracted light will be determined by the orientation and size of the facets and lands. When risers are added to the design, the size of each facet (and thus the facet area available for extraction) is in effect increased. The overall wedge-shaped extractor profile permits all or a significant portion of the total facet area to participate in extraction. Thus, inclusion of a riser and use of an overall wedge shape enables alteration of the extractor design to increase light extraction to a display. This can enable improvements in the overall size and efficiency of the device and can make better use of the available light.
In another embodiment, the plateaus of the extraction structures of the light guides have greater area than the lands separating them. Alternatively stated, a majority of the extraction structures have plateaus that are larger than the adjacent lands.
In a still further embodiment, at least some of the extraction structures have protrusions where the protrusions have at least riser segments and facet segments.
The present invention also provides point to line converters, backlights and front light guides comprising the above-described wedge-shaped light extraction device, and displays containing such extraction devices.